Data processing apparatus and method

ABSTRACT

A data processor maps input symbols to be communicated onto a predetermined number of sub-carrier signals of an Orthogonal Frequency Division Multiplexed (OFDM) symbol. The data processor includes an interleaver memory which reads-in the predetermined number of data symbols for mapping onto the OFDM sub-carrier signals. The interleaver memory reads-out the data symbols on to the OFDM sub-carriers to effect the mapping, the read-out being in a different order than the read-in, the order being determined from a set of addresses, with the effect that the data symbols are interleaved on to the sub-carrier signals. The set of addresses are generated from an address generator which comprises a linear feedback shift register and a permutation circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 12/256,746, filed Oct. 23, 2008, which claims priority ofUnited Kingdom Applications Nos. 0721269.9, filed Oct. 30, 2007,0721270.7, filed Oct. 30, 2007, 0722645.9, filed Nov. 19, 2007, and0722728.3, filed Nov. 20, 2007.

FIELD OF INVENTION

The present invention relates to data processing apparatus operable tomap input data symbols onto sub-carrier signals of Orthogonal FrequencyDivision Multiplexed (OFDM) symbols. The present invention also relatesto an address generator for use in writing symbols to/reading symbolsfrom an interleaver memory.

The present invention also relates to data processing apparatus operableto map data symbols received from a predetermined number of sub-carriersignals of OFDM symbols into an output symbol stream.

Embodiments of the present invention can provide an OFDMtransmitter/receiver.

BACKGROUND OF THE INVENTION

The Digital Video Broadcasting-Terrestrial standard (DVB-T) utilisesOrthogonal Frequency Division Multiplexing (OFDM) to communicate datarepresenting video images and sound to receivers via a broadcast radiocommunications signal. There are known to be two modes for the DVB-Tstandard which are known as the 2k and the 8k mode. The 2k mode provides2048 sub-carriers whereas the 8k mode provides 8192 sub-carriers.Similarly for the Digital Video Broadcasting-Handheld standard (DVB-H) a4k mode has been provided, in which the number of sub-carriers is 4096.

In order to improve the integrity of data communicated using DVB-T orDVB-H a symbol interleaver is provided in order to interleave input datasymbols as these symbols are mapped onto the sub-carrier signals of anOFDM symbol. Such a symbol interleaver comprises an interleaver memoryin combination with an address generator. The address generatorgenerates an address for each of the input symbols, each addressindicating one of the sub-carrier signals of the OFDM symbol onto whichthe data symbol is to be mapped. For the 2k mode and the 8k mode anarrangement has been disclosed in the DVB-T standard for generating theaddresses for the mapping. Likewise for the 4k mode of DVB-H standard,an arrangement for generating addresses for the mapping has beenprovided and an address generator for implementing this mapping isdisclosed in European Patent application 04251667.4. The addressgenerator comprises a linear feed back shift register which is operableto generate a pseudo random bit sequence and a permutation circuit. Thepermutation circuit permutes the order of the content of the linear feedback shift register in order to generate an address. The addressprovides an indication of a memory location of the interleaver memoryfor writing the input data symbol into or reading the input data symbolout from the interleaver memory for mapping onto one of the sub-carriersignal of the OFDM symbol. Similarly, an address generator in thereceiver is arranged to generate addresses of the interleaver memory forwriting the received data symbols into or reading the data symbols outfrom the interleaver memory to form an output data stream.

In accordance with a further development of the Digital VideoBroadcasting-Terrestrial broadcasting standard, known as DVB-T2 therehas been proposed that further modes for communicating data be provided.

SUMMARY OF INVENTION

According to an aspect of the present invention there is provided a dataprocessing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbol. The dataprocessing apparatus comprises an interleaver operable to read-into aninterleaver memory the predetermined number of input data symbols formapping onto the OFDM sub-carrier signals, and to read-out of theinterleaver memory the input data symbols for the OFDM sub-carriers toeffect the mapping. The read-out is in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the input data symbols are interleaved on the sub-carriersignals. The set of addresses is determined by an address generator, anaddress being generated for each of the input symbols to indicate one ofthe sub-carrier signals onto which the data symbol is to be mapped.

The address generator comprises a linear feedback shift registerincluding a predetermined number of register stages and is operable togenerate a pseudo-random bit sequence in accordance with a generatorpolynomial, and a permutation circuit and a control unit. Thepermutation circuit is operable to receive the content of the shiftregister stages and to permute the bits present in the register stagesin accordance with a permutation code to form an address of one of theOFDM sub-carriers.

The control unit is operable in combination with an address checkcircuit to re-generate an address when a generated address exceeds apredetermined maximum valid address. The data processing apparatus ischaracterised in that the predetermined maximum valid address is lessthan one thousand and twenty four, the linear feedback shift registerhas nine register stages with a generator polynomial for the linearfeedback shift register of R′_(i)[8]=R′_(i-1)[0]⊕R′_(i-1)[4], and thepermutation code forms with an additional bit, a ten bit addressR_(i)[n] for the i-th data symbol from the bit present in the n-thregister stage R′_(i)[n] in accordance with the table:

R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 4 3 2 1 0 5 67 8

Although it is known within the DVB-T standard to provide the 2k modeand the 8k mode, and the DVB-H standard provides a 4k mode, there hasbeen proposed to provide a 1k mode for DVB-T2. Whilst the 8k modeprovides an arrangement for establishing a single frequency network withsufficient guard periods to accommodate larger propagation delaysbetween DVB transmitters, the 2k mode is known to provide an advantagein mobile applications. This is because the 2k symbol period is only onequarter of the 8k symbol period, allowing the channel estimation to bemore frequently updated allowing the receiver to track the timevariation of the channel due to doppler and other effects moreaccurately. The 2k mode is therefore advantageous for mobileapplications. However, it has been proposed that an OFDM communicationssystem according to the DVB-T2 standard is required to communicate ineven harsher environments, which require a receiver to perform withgreater time variations in the communications channel, such as mobileapplications. Therefore a 1k mode has been proposed, although with the1k mode a multiple frequency network will be required complicating anarrangement of transmitters to provide a broadcast system. However, inorder to provide the 1k mode, a symbol interleaver must be provided formapping the input data symbols onto the sub-carrier signals of the OFDMsymbol.

Embodiments of the present invention can provide a data processingapparatus operable as a symbol interleaver for mapping data symbols tobe communicated on an OFDM symbol, having substantially one thousandsub-carrier signals. In one embodiment the number of sub-carrier signalsmaybe a value substantially between seven hundred and one thousand andtwenty four. Furthermore, the OFDM symbols may include pilotsub-carriers, which are arranged to carry known symbols, and thepredetermined maximum valid address depends on a number of the pilotsub-carrier symbols present in the OFDM symbol. As such the 1k mode canbe provided for example for a DVB standard, such as DVB-T2, DVB-T orDVB-H.

Mapping data symbols to be transmitted onto the sub-carrier signals ofan OFDM symbol, where the number of sub-carrier signals is approximatelyone thousand, represents a technical problem requiring simulationanalysis and testing to establish an appropriate generator polynomialfor the linear feedback shift register and the permutation order. Thisis because the mapping requires that the symbols are interleaved ontothe sub-carrier signals with the effect that successive symbols from theinput data stream are separated in frequency by a greatest possibleamount in order to optimise the performance of error correction codingschemes.

Error correction coding schemes such as Low Density ParityCheck/Bose-Chaudhuri-Hocquenghem (LDPC/BCH) coding, which has beenproposed for DVB-T2 perform better when noise and degradation of thesymbol values resulting from communication is un-correlated. Terrestrialbroadcast channels may suffer from correlated fading in both the timeand the frequency domains. As such by separating encoded symbols on todifferent sub-carrier signals of the OFDM symbol by as much as possible,the performance of error correction coding schemes can be increased.

As will be explained, it has been discovered from simulation performanceanalysis that the generator polynomial for the linear feed back shiftregister in combination with the permutation circuit order indicatedabove, provides a good performance. Furthermore, by providing anarrangement which can implement address generating for each of the 2kmode, the 4k mode and the 8k mode by changing the taps of the generatorpolynomial for the linear feed back shift register and the permutationorder, a cost effective implementation of the symbol interleaver for the1k mode can be provided. Furthermore, a transmitter and a receiver canbe changed between the 1k mode, 2k mode, 4k mode, the 8k mode and the16k mode by changing the generator polynomial and the permutationorders. This can be effected in software (or by the embedded signalling)whereby a flexible implementation is provided.

The additional bit, which is used to form the address from the contentof the linear feedback shift register, may be produced by a togglecircuit, which changes from 1 to 0 for each address, so as to reduce alikelihood that if an address exceeds the predetermined maximum validaddress, then the next address will be a valid address. In one examplethe additional bit is the most significant bit.

In one example the above permutation code is used to generate theaddresses for performing the interleaving for successive OFDM symbols.In other examples, the above permutation code is one of a plurality ofpermutation codes which are changed so as to reduce a possibility thatsuccessive or data bits which are close in order in an input data streamare mapped onto the same sub-carrier of an OFDM symbol. In one example,a different permutation code is used for performing the interleavingbetween successive OFDM symbols. The use of different permutation codesfor successive OFDM symbols can provide an advantage where the dataprocessing apparatus is operable to interleave the input data symbolsonto the sub-carrier signals of both even and odd OFDM symbols for atransmitter only by reading in the data symbols into the memory in asequential order and reading out the data symbols from the memory inaccordance with the set of addresses generated by the address generatorand for a receiver only by reading in the data symbols into the memoryin accordance with the set of addresses generated by the addressgenerator and reading out the data symbols from the memory in asequential order.

Various aspects and features of the present invention are defined in theappended claims. Further aspects of the present invention include a dataprocessing apparatus and method operable to map symbols received from apredetermined number of sub-carrier signals of an Orthogonal FrequencyDivision Multiplexed (OFDM) symbol into an output symbol stream, as wellas a transmitter and a receiver.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, wherein likeparts are provided with corresponding reference numerals, and in which:

FIG. 1 is a schematic block diagram of a Coded OFDM transmitter whichmay be used, for example, with the DVB-T2 standard;

FIG. 2 is a schematic block diagram of parts of the transmitter shown inFIG. 1 in which a symbol mapper and a frame builder illustrate theoperation of an interleaver;

FIG. 3 is a schematic block diagram of the symbol interleaver shown inFIG. 2;

FIG. 4 is a schematic block diagram of an interleaver memory shown inFIG. 3 and the corresponding symbol de-interleaver in the receiver;

FIG. 5 is a schematic block diagram of an address generator shown inFIG. 3 for the 1k mode;

FIG. 6( a) is diagram illustrating results for an interleaver using theaddress generator shown in FIG. 5 for even symbols and FIG. 6( b) is adiagram illustrating design simulation results for odd symbols, whereasFIG. 6( c) is a diagram illustrating comparative results for an addressgenerator using a different permutation code for even and FIG. 6( d) isa corresponding diagram for odd symbols;

FIG. 7 is a schematic block diagram of a Coded OFDM receiver which maybe used, for example, with the DVB-T2 standard;

FIG. 8 is a schematic block diagram of a symbol de-interleaver whichappears in FIG. 7;

FIG. 9( a) is diagram illustrating results for an interleaver for evenOFDM symbols and FIG. 9( b) is a diagram illustrating results for oddOFDM symbols;

FIG. 10 provides a schematic block diagram of the symbol interleavershown in FIG. 3, illustrating an operating mode in which interleaving isperformed in accordance with an odd interleaving mode only; and

FIG. 11 provides a schematic block diagram of the symbol de-interleavershown in FIG. 8, illustrating the operating mode in which interleavingis performed in accordance with the odd interleaving mode only.

DESCRIPTION OF PREFERRED EMBODIMENTS

It has been proposed that the number of modes, which are availablewithin the DVB-T2 standard should be extended to include a 1k mode, a16k mode and a 32k mode. The following description is provided toillustrate the operation of a symbol interleaver in accordance with thepresent technique, although it will be appreciated that the symbolinterleaver can be used with other modes and other DVB standards.

FIG. 1 provides an example block diagram of a Coded OFDM transmitterwhich may be used for example to transmit video images and audio signalsin accordance with the DVB-T2 standard. In FIG. 1 a program sourcegenerates data to be transmitted by the Coded Orthogonal FrequencyDivision Multiplexing (COFDM) transmitter. A video coder 2, and audiocoder 4 and a data coder 6 generate video, audio and other data to betransmitted which are fed to a program multiplexer 10. The output of theprogram multiplexer 10 forms a multiplexed stream with other informationrequired to communicate the video, audio and other data. The multiplexer10 provides a stream on a connecting channel 12. There may be many suchmultiplexed streams which are fed into different branches A, B etc. Forsimplicity, only branch A will be described.

As shown in FIG. 1 a COFDM transmitter receives the stream at amultiplexer adaptation and energy dispersal block 22. The multiplexeradaptation and energy dispersal block 22 randomizes the data and feedsthe appropriate data to a forward error correction encoder 24 whichperforms error correction encoding of the stream. A bit interleaver 26is provided to interleave the encoded data bits which for the example ofDVB-T2 is the LDCP/BCH encoder output. The output from the bitinterleaver 26 is fed to a bit into constellation mapper 28, which mapsgroups of bits onto a constellation point, which is to be used forconveying the encoded data bits. The outputs from the bit intoconstellation mapper 28 are constellation point labels that representreal and imaginary components. The constellation point labels representdata symbols formed from two or more bits depending on the modulationscheme used. These will be referred to as data cells. These data cellsare passed through a time-interleaver 30 whose effect is to interleaverdata cells resulting from multiple LDPC code words.

The data cells are received by a frame builder 32, with data cellsproduced by branch B etc in FIG. 1, via other channels 31. The framebuilder 32 then forms many data cells into sequences to be conveyed onCOFDM symbols, where a COFDM symbol comprises a number of data cells,each data cell being mapped onto one of the sub-carriers. The number ofsub-carriers will depend on the mode of operation of the system, whichmay include one of 1k, 2k, 4k, 8k, 16k or 32k, each of which provides adifferent number of sub-carriers according, for example to the followingtable:

Number of Sub-carriers Adapted from DVB-T/H Mode Sub-carriers 1k 756 2k1512 4k 3024 8k 6048 16k  12096 32k  24192

Thus in one example, the number of sub-carriers for the 1k mode is sevenhundred and fifty six. For the DVB-T2 system, the number of sub-carriersper OFDM symbol can vary depending upon the number of pilot and otherreserved carriers. Thus, in DVB-T2, unlike in DVB-T, the number ofsub-carriers for carrying data is not fixed. Broadcasters can select oneof the operating modes from 1k, 2k, 4k, 8k, 16k, 32k each providing arange of sub-carriers for data per OFDM symbol, the maximum availablefor each of these modes being 1024, 2048, 4096, 8192, 16384, 32768respectively. In DVB-T2 a physical layer frame is composed of many OFDMsymbols. Typically the frame starts with one or more preamble or P2 OFDMsymbols, which are then followed by a number payload carrying OFDMsymbols. The end of the physical layer frame is marked by a frameclosing symbols. For each operating mode, the number of sub-carriers maybe different for each type of symbol. Furthermore, this may vary foreach according to whether bandwidth extension is selected, whether tonereservation is enabled and according to which pilot sub-carrier patternhas been selected. As such a generalisation to a specific number ofsub-carriers per OFDM symbol is difficult. However, the frequencyinterleaver for each mode can interleave any symbol whose number ofsub-carriers is smaller than or the same as the maximum available numberof sub-carriers for the given mode. For example, in the 1k mode, theinterleaver would work for symbols with the number of sub-carriers beingless than or equal to 1024 and for 16k mode, with the number ofsub-carriers being less than or equal to 16384.

The sequence of data cells to be carried in each COFDM symbol is thenpassed to the symbol interleaver 33. The COFDM symbol is then generatedby a COFDM symbol builder block 37 which introduces pilot andsynchronising signals fed from a pilot and embedded signal former 36. AnOFDM modulator 38 then forms the OFDM symbol in the time domain which isfed to a guard insertion processor 40 for generating a guard intervalbetween symbols, and then to a digital to analogue convertor 42 andfinally to an RF amplifier within an RF front end 44 for eventualbroadcast by the COFDM transmitter from an antenna 46.

Providing a 1k Mode

To create a new 1K mode, several elements are to be defined, one ofwhich is the 1K symbol interleaver 33. The bit to constellation mapper28, symbol interleaver 33 and the frame builder 32 are shown in moredetail in FIG. 2.

As explained above, the present invention provides a facility forproviding a quasi-optimal mapping of the data symbols onto the OFDMsub-carrier signals. According to the example technique the symbolinterleaver is provided to effect the optimal mapping of input datasymbols onto COFDM sub-carrier signals in accordance with a permutationcode and generator polynomial, which has been verified by simulationanalysis.

As shown in FIG. 2 a more detailed example illustration of the bit tosymbol constellation mapper 28 and the frame builder 32 is provided toillustrate an example embodiment of the present technique. Data bitsreceived from the bit interleaver 26 via a channel 62 are grouped intosets of bits to be mapped onto a data cell, in accordance with a numberof bits per symbol provided by the modulation scheme. The groups ofbits, which forms a data word, are fed in parallel via data channels 64the a mapping processor 66. The mapping processor 66 then selects one ofthe data symbols, in accordance with a pre-assigned mapping. Theconstellation point, is represented by a real and an imaginary componentthat is provided to the output channel 29 as one of a set of inputs tothe frame builder 32.

The frame builder 32 receives the data cells from the bit toconstellation mapper 28 through channel 29, together with data cellsfrom the other channels 31. After building a frame of many COFDM cellsequences, the cells of each COFDM symbol are then written into aninterleaver memory 100 and read out of the interleaver memory 100 inaccordance with write addresses and read addresses generated by anaddress generator 102. According to the write-in and read-out order,interleaving of the data cells is achieved, by generating appropriateaddresses. The operation of the address generator 102 and theinterleaver memory 100 will be described in more detail shortly withreference to FIGS. 3, 4 and 5. The interleaved data cells are thencombined with pilot and synchronisation symbols received from the pilotand embedded signalling former 36 into an OFDM symbol builder 37, toform the COFDM symbol, which is fed to the OFDM modulator 38 asexplained above.

Interleaver

FIG. 3 provides an example of parts of the symbol interleaver 33, whichillustrates the present technique for interleaving symbols. In FIG. 3the input data cells from the frame builder 32 are written into theinterleaver memory 100. The data cells are written into the interleavermemory 100 according to a write address fed from the address generator102 on channel 104, and read out from the interleaver memory 100according to a read address fed from the address generator 102 on achannel 106. The address generator 102 generates the write address andthe read address as explained below, depending on whether the COFDMsymbol is odd or even, which is identified from a signal fed from achannel 108, and depending on a selected mode, which is identified froma signal fed from a channel 110. As explained, the mode can be one of a1k mode, 2k mode, 4k mode, 8k mode, 16k mode or a 32k mode. As explainedbelow, the write address and the read address are generated differentlyfor odd and even symbols as explained with reference to FIG. 4, whichprovides an example implementation of the interleaver memory 100.

In the example shown in FIG. 4, the interleaver memory is shown tocomprise an upper part 100 illustrating the operation of the interleavermemory in the transmitter and a lower part 340, which illustrates theoperation of the de-interleaver memory in the receiver. The interleaver100 and the de-interleaver 340 are shown together in FIG. 4 in order tofacilitate understanding of their operation. As shown in FIG. 4 arepresentation of the communication between the interleaver 100 and thede-interleaver 340 via other devices and via a transmission channel hasbeen simplified and represented as a section 140 between the interleaver100 and the de-interleaver 340. The operation of the interleaver 100 isdescribed in the following paragraphs:

Although FIG. 4 provides an illustration of only four input data cellsonto an example of four sub-carrier signals of a COFDM symbol, it willbe appreciated that the technique illustrated in FIG. 4 can be extendedto a larger number of sub-carriers such as 756 for the 1k mode 1512 forthe 2k mode, 3024 for the 4k mode and 6048 for the 8k mode, 12096 forthe 16k mode and 24192 for the 32k mode.

The input and output addressing of the interleaver memory 100 shown inFIG. 4 is shown for odd and even symbols. For an even COFDM symbol thedata cells are taken from the input channel and written into theinterleaver memory 124.1 in accordance with a sequence of addresses 120generated for each COFDM symbol by the address generator 102. The writeaddresses are applied for the even symbol so that as illustratedinterleaving is effected by the shuffling of the write-in addresses.Therefore, for each interleaved symbol y(h(q))=y′(q).

For odd symbols the same interleaver memory 124.2 is used. However, asshown in FIG. 4 for the odd symbol the write-in order 132 is in the sameaddress sequence used to read out the previous even symbol 126. Thisfeature allows the odd and even symbol interleaver implementations toonly use one interleaver memory 100 provided the read-out operation fora given address is performed before the write-in operation. The datacells written into the interleaver memory 124.2 during odd symbols arethen read out in a sequence 134 generated by the address generator 102for the next even COFDM symbol and so on. Thus only one address isgenerated per symbol, with the read-in and write-out for the odd/evenCOFDM symbol being performed contemporaneously.

In summary, as represented in FIG. 4, once the set of addresses H(q) hasbeen calculated for all active sub-carriers, the input vector Y′=(y′₀,y′₁, y′₂, . . . y′_(Nmax-1)) is processed to produce the interleavedvector Y=(y₀, y₁, y₂, . . . y_(Nmax-1)) defined by:

y_(H(q))=y′_(q) for even symbols for q=0, . . . , N_(max)−1

y_(q)=y′_(H(q)) for odd symbols for q=0, . . . , N_(max)−1

In other words, for even OFDM symbols the input words are written in apermutated way into a memory and read back in a sequential way, whereasfor odd symbols, they are written sequentially and read back permutated.In the above case, the permutation H(q) is defined by the followingtable:

TABLE 1 permutation for simple case where Nmax = 4

As shown in FIG. 4, the de-interleaver 340 operates to reverse theinterleaving applied by the interleaver 100, by applying the same set ofaddresses as generated by an equivalent address generator, but applyingthe write-in and read-out addresses in reverse. As such, for evensymbols, the write-in addresses 342 are in sequential order, whereas theread out address 344 are provided by the address generator.Correspondingly, for the odd symbols, the write-in order 346 isdetermined from the set of addresses generated by the address generator,whereas read out 348 is in sequential order.

Address Generation for the 1k Mode

A schematic block diagram of the algorithm used to generate thepermutation function H(q) is represented in FIG. 5 for the 1K mode.

An implementation of the address generator 102 for the 1k mode is shownin FIG. 5. In FIG. 5 a linear feed back shift register is formed by nineregister stages 200 and a xor-gate 202 which is connected to the stagesof the shift register 200 in accordance with a generator polynomial.Therefore, in accordance with the content of the shift register 200 anext bit of the shift register is provided from the output of thexor-gate 202 by xoring the content of shift register R[0] and registerstage R[4] according to the generator polynomial:

R′ _(i)[8]=R′ _(i-1)[0]⊕R′ _(i-1)[4]

According to the generator polynomial a pseudo random bit sequence isgenerated from the content of the shift register 200. However, in orderto generate an address for the 1k mode as illustrated, a permutationcircuit 210 is provided which effectively permutes the order of the bitswithin the shift register 200 from an order R′_(i)[n] to an orderR_(i)[n] at the output of the permutation circuit 210. Nine bits fromthe output of the permutation circuit 210 are then fed on a connectingchannel 212 to which is added a most significant bit via a channel 214which is provided by a toggle circuit 218. A ten bit address istherefore generated on channel 212. However, in order to ensure theauthenticity of an address, an address check circuit 216 analyses thegenerated address to determine whether it exceeds a predeterminedmaximum value. The predetermined maximum value may correspond to themaximum number of sub-carrier signals, which are available for datasymbols within the COFDM symbol, available for the mode which is beingused. However, the interleaver for the 1k mode may also be used forother modes, so that the address generator 102 may also be used for the2k mode, 4k mode, 8k mode, 16k mode and the 32k mode, by adjustingaccordingly the number of the maximum valid address.

If the generated address exceeds the predetermined maximum value then acontrol signal is generated by the address check unit 216 and fed via aconnecting channel 220 to a control unit 224. If the generated addressexceeds the predetermined maximum value then this address is rejectedand a new address regenerated for the particular symbol.

For the 1k mode, an (N_(r)−1) bit word R′_(i) is defined, withN_(r)=log₂ M_(max), where M_(max)=1024 using a LFSR (Linear FeedbackShift Register).

The polynomial used to generate this sequence is:

1K mode: R′ _(i)[8]=R′ _(i-1)[0]⊕R′ _(i-1)[4]

where i varies from 0 to M_(max)−1

Once one R′_(i), word has been generated, the R′_(i), word goes througha permutation to produce another (N_(r)−1) bit word called R_(i). R_(i)is derived from R′_(i) by the bit permutations given as follows:

R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 4 3 2 1 0 5 67 8

Bit permutation for the 1K mode

As an example, this means that for the mode 1K, the bit number 8 ofR′_(i) is sent into bit position number 4 of R_(i).

The address H(q) is then derived from R, through the following equation:

${H(q)} = {{\left( {i\; {mod}\; 2} \right) \cdot 2^{N_{t} - 1}} + {\sum\limits_{j = 0}^{N_{t} - 2}{{R_{i}(j)} \cdot 2^{j}}}}$

The (i mod2)·2^(N) ^(r) ⁻¹ part of the above equation is represented inFIG. 5 by the toggle block T 218.

An address check is then performed on H(q) to verify that the generatedaddress is within the range of acceptable addresses: if (H(q)<N_(max)),where N_(max)=756 in the 1K mode for example, then the address is valid.If the address is not valid, the control unit is informed and it willtry to generate a new H(q) by incrementing the index i.

The role of the toggle block is to make sure that we do not generate anaddress exceeding N_(max) twice in a row. In effect, if an exceedingvalue was generated, this means that the MSB (i.e. the toggle bit) ofthe address H(q) was one. So the next value generated will have a MSBset to zero, insuring to produce a valid address.

The following equations sum up the overall behaviour and help tounderstand the loop structure of this algorithm:

q = 0; for  (i = 0; i < M_(ma x); i = i + 1) $\begin{Bmatrix}{{{H(q)} = {{\left( {i\; {mod}\; 2} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}};} \\{{{{if}\mspace{14mu} \left( {{H(q)} < N_{{ma}\; x}} \right)q} = {q + 1}};}\end{Bmatrix}$

As will be explained shortly, in one example of the address generator,the above mentioned permutation code is used for generating addressesfor all OFDM symbols. In another example, the permutation codes may bechanged between symbols, with the effect that a set of permutation codesare cycled through for successive OFMD symbols. To this end, the controllines 108, 110 providing an indication as to whether the OFDM symbol isodd or even and the current mode are used to select the permutationcode. This example mode in which a plurality of permutation codes arecycled through is particularly appropriate for the example in which theodd interleaver only is used, which will be explained later. A signalindicating that a different permutation code should be used is providedvia a control channel. In one example the possible permutation codes arepre-stored in the permutation code circuit 210. In another example, thecontrol unit 224 supplies the new permutation code to be used for anOFDM symbol.

Analysis Supporting the Address Generator for the 1k Mode

The selection of the polynomial generator and the permutation codeexplained above for the address generator 102 for the 1k mode has beenidentified following simulation analysis of the relative performance ofthe interleaver. The relative performance of the interleaver has beenevaluated using a relative ability of the interleaver to separatesuccessive symbols or an “interleaving quality”. As mentioned above,effectively the interleaving must perform for both odd and even symbols,in order to use a single interleaver memory. The relative measure of theinterleaver quality is determined by defining a distance D (in number ofsub-carriers). A criterion C is chosen to identify a number ofsub-carriers that are at distance ≦D at the output of the interleaverthat were at distance D at the input of the interleaver, the number ofsub-carriers for each distance ≦D then being weighted with respect tothe relative distance. The criterion C is evaluated for both odd andeven COFDM symbols. Minimising C produces a superior qualityinterleaver.

$C = {{\sum\limits_{1}^{d = D}{{N_{even}(d)}/d}} + {\sum\limits_{1}^{d = D}{{N_{odd}(d)}/d}}}$

where: N_(even)(d) and N_(odd)(d) are number of sub-carriers in an evenand odd symbol respectively at the output of the interleaver that remainwithin d sub-carrier spacing of each other.

Analysis of the interleaver identified above for the 1k mode for a valueof D=5 is shown in FIG. 6( a) for the even COFDM symbols and in FIG. 6(b) for the odd COFDM symbol. According to the above analysis, the valueof C for the permutation code identified above for the 1k mode produceda value of C=24, that the weighted number of sub-carriers with symbolswhich are separated by five or less in the output according to the aboveequation was 24.

A corresponding analysis is provided for an alternative permutation codefor even COFDM symbols in FIG. 6( c) for odd COFDM symbols in FIG. 6(d). As can be seen in comparison to the results illustrated in FIGS. 6(a) and 6(b), there are more components present which represent symbolsseparated by small distances such as D=1, and D=2, when compared withthe results shown in FIGS. 6( a) and 6(b), illustrating that thepermutation code identified above for the 1k mode symbol interleaverproduces a superior quality interleaver.

Alternative Permutation Codes

The following ten alternative possible codes ([n]R_(i) bit positions,where n=1 to 10) have been found to provide a symbol interleaver with agood quality as determined by the criterion C identified above.

R′_(i) bit positions 8 7 6 5 4 3 2 1 0 [1]R_(i) bit positions 5 3 2 1 06 7 4 8 [2]R_(i) bit positions 4 3 2 1 0 6 7 5 8 [3]R_(i) bit positions4 3 2 1 0 5 7 6 8 [4]R_(i) bit positions 3 2 1 5 0 6 4 7 8 [5]R_(i) bitpositions 4 2 3 0 1 7 5 8 6 [6]R_(i) bit positions 4 2 3 0 1 5 7 8 6[7]R_(i) bit positions 4 2 3 0 1 5 6 8 7 [8]R_(i) bit positions 3 2 5 01 4 7 8 6 [9]R_(i) bit positions 4 2 3 0 1 5 7 8 6 [10]R_(i) bitpositions 4 3 2 1 0 5 6 7 8

Bit permutation for the 1K mode

Receiver

FIG. 7 provides an example illustration of a receiver which may be usedwith the present technique. As shown in FIG. 7, a COFDM signal isreceived by an antenna 300 and detected by a tuner 302 and convertedinto a digital form by an analogue-to-digital converter 304. A guardinterval removal processor 306 removes the guard interval from areceived COFDM symbol, before the data is recovered from the COFDMsymbol using a Fast Fourier Transform (FFT) processor 308 in combinationwith a channel estimator and correction 310 in co-operation with aembedded-signalling decoding unit 311, in accordance with knowntechniques. The demodulated data is recovered from a mapper 312 and fedto a symbol de-interleaver 314, which operates to effect the reversemapping of the received data symbol to re-generate an output data streamwith the data de-interleaved.

The symbol de-interleaver 314 is formed from a data processing apparatusas shown in FIG. 8 with an interleaver memory 540 and an addressgenerator 542. The interleaver memory is as shown in FIG. 4 and operatesas already explained above to effect de-interleaving by utilising setsof addresses generated by the address generator 542. The addressgenerator 542 is formed as shown in FIG. 8 and is arranged to generatecorresponding addresses to map the data symbols recovered from eachCOFDM sub-carrier signals into an output data stream.

The remaining parts of the COFDM receiver shown in FIG. 7 including theBit De-interleaver are provided to effect error correction decoding 318to correct errors and recover an estimate of the source data.

One advantage provided by the present technique for both the receiverand the transmitter is that a symbol interleaver and a symbolde-interleaver operating in the receivers and transmitters can beswitched between the 1k, 2k, 4k, 8k, 16k and the 32k mode by changingthe generator polynomials and the permutation order. Hence the addressgenerator 542 shown in FIG. 8 includes an input 544, providing anindication of the mode as well as an input 546 indicating whether thereare odd/even COFDM symbols. A flexible implementation is therebyprovided because a symbol interleaver and de-interleaver can be formedas shown in FIGS. 3 and 8, with an address generator as illustrated ineither of FIG. 5. The address generator can therefore be adapted to thedifferent modes by changing to the generator polynomials and thepermutation orders indicated for each of the modes. For example, thiscan be effected using a software change. Alternatively, in otherembodiments, an embedded signal indicating the mode of the DVB-T2transmission can be detected in the receiver in the embedded-signallingprocessing unit 311 and used to configure automatically the symbolde-interleaver in accordance with the detected mode.

Optimal Use of Odd Interleavers

As shown in FIG. 4, two symbol interleaving processes, one for evenCOFDM symbols and one for odd COFDM symbols allows the amount of memoryused during interleaving to be reduced. In the example shown in FIG. 4,the write in order for the odd symbol is the same as the read out orderfor the even symbol therefore, while an odd symbol is being read fromthe memory, an even symbol can be written to the location just readfrom; subsequently, when that even symbol is read from the memory, thefollowing odd symbol can be written to the location just read from.

As mentioned above, during an experimental analysis of the performanceof the interleavers (using criterion C as defined above) and for exampleshown in FIG. 9( a) and FIG. 9( b) it has been discovered that theinterleaving schemes designed for the 2k and 8k symbol interleavers forDVB-T and the 4k symbol interleaver for DVB-H work better for oddsymbols than even symbols. Thus from performance evaluation results ofthe interleavers, for example, as illustrated by FIGS. 9( a) and 9(b)have revealed that the odd interleavers work better than the eveninterleavers. This can be seen by comparing FIG. 9( a) which showsresults for an interleaver for even symbols and FIG. 9( b) illustratingresults for odd symbols: it can be seen that the average distance at theinterleaver output of sub-carriers that were adjacent at the interleaverinput is greater for an interleaver for odd symbols than an interleaverfor even symbols.

As will be understood, the amount of interleaver memory required toimplement a symbol interleaver is dependent on the number of datasymbols to be mapped onto the COFDM carrier symbols. Thus a 16k modesymbol interleaver requires half the memory required to implement a 32kmode symbol interleaver and similarly, the amount of memory required toimplement an 8k symbol interleaver is half that required to implement a16k interleaver. Therefore a transmitter or receiver which is arrangedto implement a symbol interleaver of a mode, which sets the maximumnumber of data symbols which can be carried per OFDM symbol, then thatreceiver or transmitter will include sufficient memory to implement twoodd interleaving processes for any other mode, which provides half orsmaller than half the number of sub-carriers per OFDM symbol in thatgiven maximum mode. For example a receiver or transmitter including a32K interleaver will have enough memory to accommodate two 16K oddinterleaving processes each with their own 16K memory.

Therefore, in order to exploit the better performance of the oddinterleaving processes, a symbol interleaver capable of accommodatingmultiple modulation modes can be arranged so that only an odd symbolinterleaving process is used if in a mode which comprises half or lessthan half of the number of sub-carriers in a maximum mode, whichrepresents the maximum number of sub-carriers per OFDM symbol. Thismaximum mode therefore sets the maximum memory size. For example, in atransmitter/receiver capable of the 32K mode, when operating in a modewith fewer carriers (i.e. 16K, 8K, 4K or 1K) then rather than employingseparate odd and even symbol interleaving processes, two oddinterleavers would be used.

An illustration of an adaptation of the symbol interleaver 33 which isshown in FIG. 3 when interleaving input data symbols onto thesub-carriers of OFDM symbols in the odd interleaving mode only is shownin FIG. 10. The symbol interleaver 33.1 corresponds exactly to thesymbol interleaver 33 as shown in FIG. 3, except that the addressgenerator 102 is adapted to perform the odd interleaving process only.For the example shown in FIG. 10, the symbol interleaver 33.1 isoperating in a mode where the number of data symbols which can becarried per OFDM symbol is less than half of the maximum number whichcan be carried by an OFDM symbol in an operating mode with the largestnumber of sub-carriers per OFDM symbol. As such, the symbol interleaver33.1 has been arranged to partition the interleaver memory 100. For thepresent illustration shown in FIG. 10 the interleaver memory then 100 isdivided into two parts 401, 402. As an illustration of the symbolinterleaver 33.1 operating in a mode in which data symbols are mappedonto the OFDM symbols using the odd interleaving process, FIG. 10provides an expanded view of each half of the interleaver memory 401,402. The expanded provides an illustration of the odd interleaving modeas represented for the transmitter side for four symbols A, B, C, Dreproduced from FIG. 4. Thus as shown in FIG. 10, for successive sets offirst and second data symbols, the data symbols are written into theinterleaver memory 401, 402 in a sequential order and read out inaccordance with addresses generated by the address generator 102 in apermuted order in accordance with the addresses generated by the addressgenerator as previously explained. Thus as illustrated in FIG. 10, sincean odd interleaving process is being performed for successive sets offirst and second sets of data symbols, the interleaver memory must bepartitioned into two parts. Symbols from a first set of data symbols arewritten into a first half of the interleaver memory 401, and symbolsfrom a second set of data symbols are written into a second part of theinterleaver memory 402, because the symbol interleaver is no longer ableto reuse the same parts of the symbol interleaver memory as can beaccommodated when operating in an odd and even mode of interleaving.

A corresponding example of the interleaver in the receiver, whichappears in FIG. 8 but adapted to operate with an odd interleavingprocess only is shown in FIG. 11. As shown in FIG. 11 the interleavermemory 540 is divided into two halves 410, 412 and the address generator542 is adapted to write data symbols into the interleaver memory andread data symbols from the interleaver memory into respective parts ofthe memory 410, 412 for successive sets of data symbols to implement anodd interleaving process only. Therefore, in correspondence withrepresentation shown in FIG. 10, FIG. 11 shows the mapping of theinterleaving process which is performed at the receiver and illustratedin FIG. 4 as an expanded view operating for both the first and secondhalves of the interleaving memory 410, 412. Thus a first set of datasymbols are written into a first part of the interleaver memory 410 in apermuted order defined in accordance with the addresses generated by theaddress generator 542 as illustrated by the order of writing in the datasymbols which provides a write sequence of 1, 3, 0, 2. As illustratedthe data symbols are then read out of the first part of the interleavermemory 410 in a sequential order thus recovering the original sequenceA, B, C, D.

Correspondingly, a second subsequent set of data symbols which arerecovered from a successive OFDM symbol are written into the second halfof the interleaver memory 412 in accordance with the addresses generatedby the address generator 542 in a permuted order and read out into theoutput data stream in a sequential order.

In one example the addresses generated for a first set of data symbolsto write into the first half of the interleaver memory 410 can be reusedto write a second subsequent set of data symbols into the interleavermemory 412. Correspondingly, the transmitter may also reuse addressesgenerated for one half of the interleaver for a first set of datasymbols for reading out a second set of data symbols which have beenwritten into the second half of the memory in sequential order.

Odd Interleaver with Offset

The performance of an interleaver, which uses two odd interleavers couldbe further improved by using a sequence of odd only interleavers ratherthan a single odd only interleaver, so that any bit of data input to theinterleave does not always modulate the same carrier in the OFDM symbol.

A sequence of odd only interleavers could be realised by either:

-   -   adding an offset to the interleaver address modulo the number of        data carriers, or    -   using a sequence of permutations in the interleaver

Adding an Offset

Adding an offset to the interleaver address modulo the number of datacarriers effectively shifts and wraps-round the OFDM symbol so that anybit of data input to the interleaver does not always modulate the samecarrier in the OFDM symbol. Thus the address generator, could optionallyinclude an offset generator, which generates an offset in an addressgenerated by the address generator on the output channel H(q).

The offset would change each symbol. For example, this offset couldprovide be a cyclic sequence. This cyclic sequence could be, forexample, of length 4 and could consist of, for example, prime numbers.For example, such a sequence could be:

0, 41, 97, 157

Furthermore, the offset may be a random sequence, which may be generatedby another address generator from a similar OFDM symbol interleaver ormay be generated by some other means.

Using a Sequence of Permutations

As shown in FIG. 5, a control line extends from the control unit of theaddress generator to the permutation circuit. As mentioned above, in oneexample the address generator can apply a different permutation codefrom a set of permutation codes for successive OFDM symbols. Using asequence of permutations in the interleaver address generator reduces alikelihood that any bit of data input to the interleaver does not alwaysmodulate the same sub-carrier in the OFDM symbol.

For example, this could be a cyclic sequence, so that a differentpermutation code in a set of permutation codes in a sequence is used forsuccessive OFDM symbols and then repeated. This cyclic sequence couldbe, for example, of length two or four. For the example of the 16Ksymbol interleaver a sequence of two permutation codes which are cycledthrough per OFDM symbol could be for example:

8 4 3 2 0 11 1 5 12 10 6 7 9 7 9 5 3 11 1 4 0 2 12 10 8 6

whereas a sequence of four permutation codes could be:

8 4 3 2 0 11 1 5 12 10 6 7 9 7 9 5 3 11 1 4 0 2 12 10 8 6 6 11 7 5 2 3 01 10 8 12 9 4 5 12 9 0 3 10 2 4 6 7 8 11 1

The switching of one permutation code to another could be effected inresponse to a change in the Odd/Even signal indicated on the controlchannel 108. In response the control unit 224 changes the permutationcode in the permutation code circuit 210 via the control line.

For the example of a 1k symbol interleaver, two permutation codes couldbe:

4 3 2 1 0 5 6 7 8 3 2 5 0 1 4 7 8 6

whereas four permutation codes could be:

4 3 2 1 0 5 6 7 8 3 2 5 0 1 4 7 8 6 7 5 3 8 2 6 1 4 0 1 6 8 2 5 3 4 0 7

Other combinations of sequences may be possible for 2k, 4k and 8kcarrier modes or indeed 0.5 k carrier mode. For example, the followingpermutation codes for each of the 0.5 k, 2k, 4k and 8k provide goodde-correlation of symbols and can be used cyclically to generate theoffset to the address generated by an address generator for each of therespective modes:

2k Mode: 0 7 5 1 8 2 6 9 3 4 * 4 8 3 2 9 0 1 5 6 7 8 3 9 0 2 1 5 7 4 6 70 4 8 3 6 9 1 5 2 4k Mode: 7 10 5 8 1 2 4 9 0 3 6 ** 6 2 7 10 8 0 3 4 19 5 9 5 4 2 3 10 1 0 6 8 7 1 4 10 3 9 7 2 6 5 0 8 8k Mode: 5 11 3 0 10 86 9 2 4 1 7 * 10 8 5 4 2 9 1 0 6 7 3 11 11 6 9 8 4 7 2 1 0 10 5 3 8 3 117 9 1 5 6 4 0 2 10

For the permutation codes indicated above, the first two could be usedin a two sequence cycle, whereas all four could be used for a foursequence cycle. In addition, some further sequences of four permutationcodes, which are cycled through to provide the offset in an addressgenerator to produce a good de-correlation in the interleaved symbols(some are common to the above) are provided below:

0.5k Mode: 3 7 4 6 1 2 0 5 4 2 5 7 3 0 1 6 5 3 6 0 4 1 2 7 6 1 0 5 2 7 43 2k Mode: 0 7 5 1 8 2 6 9 3 4 * 3 2 7 0 1 5 8 4 9 6 4 8 3 2 9 0 1 5 6 77 3 9 5 2 1 0 6 4 8 4k Mode: 7 10 5 8 1 2 4 9 0 3 6 ** 6 2 7 10 8 0 3 41 9 5 10 3 4 1 2 7 0 6 8 5 9 0 8 9 5 10 4 6 3 2 1 7 8k Mode: 5 11 3 0 108 6 9 2 4 1 7 * 8 10 7 6 0 5 2 1 3 9 4 11 11 3 6 9 2 7 4 10 5 1 0 8 10 81 7 5 6 0 11 4 2 9 3 * these are the permutations in the DVB-T standard** these are the permutations in the DVB-H standard

Examples of address generators, and corresponding interleavers, for the2k, 4k and 8k modes are disclosed in European patent application number04251667.4, which corresponds to U.S. Patent Application Publication No.2008/298487, the contents of which are incorporated herein be reference.An address generator for the 0.5 k mode are disclosed in our co-pendingUK patent application number 0722553.5, which corresponds to UKpublished application No. GB 2454722.

Various modifications may be made to the embodiments described abovewithout departing from the scope of the present invention. Inparticular, the example representation of the generator polynomial andthe permutation order which have been used to represent aspects of theinvention are not intended to be limiting and extend to equivalent formsof the generator polynomial and the permutation order.

As will be appreciated the transmitter and receiver shown in FIGS. 1 and7 respectively are provided as illustrations only and are not intendedto be limiting. For example, it will be appreciated that the position ofthe symbol interleaver and the de-interleaver with respect, for exampleto the bit interleaver and the mapper can be changed. As will beappreciated the effect of the interleaver and de-interleaver isun-changed by its relative position, although the interleaver may beinterleaving I/Q symbols instead of v-bit vectors. A correspondingchange may be made in the receiver. Accordingly the interleaver andde-interleaver may be operating on different data types, and may bepositioned differently to the position described in the exampleembodiments.

As explained above the permutation codes and generator polynomial of theinterleaver, which has been described with reference to animplementation of a particular mode, can equally be applied to othermodes, by changing the predetermined maximum allowed address inaccordance with the number of sub-carriers for that mode.

As mentioned above, embodiments of the present invention findapplication with DVB standards such as DVB-T, DVB-T2 (published as EN302 755) and DVB-H (published as ETSI EN 302 304 V1.1.1 (2004-11)),which are incorporated herein by reference. For example embodiments ofthe present invention may be used in a transmitter or receiver operatingin accordance with the DVB-H standard, in hand-held mobile terminals.The mobile terminals may be integrated with mobile telephones (whethersecond, third or higher generation) or Personal Digital Assistants orTablet PCs for example. Such mobile terminals may be capable ofreceiving DVB-H or DVB-T/T2 compatible signals inside buildings or onthe move in for example cars or trains, even at high speeds. The mobileterminals may be, for example, powered by batteries, mains electricityor low voltage DC supply or powered from a car battery. Services thatmay be provided by DVB-H may include voice, messaging, internetbrowsing, radio, still and/or moving video images, television services,interactive services, video or near-video on demand and option. Theservices might operate in combination with one another. In otherexamples embodiments of the present invention finds application with theDVB-T2 standard as specified in accordance with ETSI standard EN 302755. In other examples embodiments of the present invention findapplication with the cable transmission standard known as DVB-C2.However, it will be appreciated that the present invention is notlimited to application with DVB and may be extended to other standardsfor transmission or reception, both fixed and mobile.

1. A data processing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbol, the dataprocessing apparatus comprising an interleaver operable to read-into aninterleaver memory the predetermined number of the input data symbolsfor mapping onto the OFDM sub-carrier signals, and to read-out of theinterleaver memory the input data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,an address generator operable to generate the set of addresses, anaddress being generated for each of the input data symbols to indicateone of the sub-carrier signals onto which the data symbol is to bemapped, the address generator comprising a linear feedback shiftregister including a predetermined number of register stages and beingoperable to generate a pseudo-random bit sequence in accordance with agenerator polynomial, a permutation circuit operable to receive thecontent of the shift register stages and to permute the bits present inthe register stages in accordance with a permutation code to form anaddress of one of the OFDM sub-carriers, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address,characterised in that the predetermined maximum valid address is lessthan one thousand and twenty four, the linear feedback shift registerhas nine register stages with a generator polynomial for the linearfeedback shift register of R′_(i)[8]=R′_(i-1)[0]⊕R′_(i-1)[4], and thepermutation order forms, with an additional bit, a ten bit addressR_(i)[n] for the i-th data symbol from the bit present in the n-thregister stage R′_(i)[n] in accordance with a code defined by the table:R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 4 3 2 1 0 5 67 8


2. A data processing apparatus as claimed in claim 1, wherein thepredetermined maximum valid address is a value substantially betweenseven hundred and one thousand and twenty four.
 3. A data processingapparatus as claimed in claim 1, wherein the OFDM symbol includes pilotsub-carriers, which are arranged to carry known symbols, and thepredetermined maximum valid address depends on a number of the pilotsub-carrier symbols present in the OFDM symbol.
 4. A data processingapparatus as claimed in claim 1, wherein the interleaver memory isoperable to effect the mapping of the input data symbols onto thesub-carrier signals for even OFDM symbols by reading in the data symbolsaccording to the set of addresses generated by the address generator andreading out in a sequential order, and for odd OFDM symbols by readingin the symbols into the interleaver memory in a sequential order andreading out the data symbols from the memory in accordance with the setof addresses generated by the address generator.
 5. A data processingapparatus as claimed in claim 1, wherein the permutation circuit isoperable to change the permutation code, which permutes the order of thebits of the register stages to form the addresses from one OFDM symbolto another.
 6. A data processing apparatus as claimed in claim 5,wherein the permutation circuit is operable to cycle through a sequenceof different permutation codes for successive OFDM symbols.
 7. A dataprocessing apparatus as claimed in claim 6, wherein the sequence ofpermutation codes comprises two permutation codes, which are: R′_(i) bitpositions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 4 3 2 1 0 5 6 7 8

and R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 3 2 5 0 14 7 8 6


8. A data processing apparatus as claimed in claim 6, wherein thesub-carriers of the OFDM symbols are half or less than half a maximumnumber of sub-carriers in the OFDM symbols of any of a plurality ofoperating modes, and the input data symbols include first sets of inputdata symbols for mapping onto even OFDM symbols and second sets of inputdata symbols for mapping onto odd OFDM symbols, and the data processingapparatus is operable to interleave the input data symbols from bothfirst and second sets in accordance with an odd interleaving process,the odd interleaving process including writing the first sets of inputdata symbols into a first part of the interleaver memory in accordancewith a sequential order of the first sets of input data symbols, readingout the first sets of input data symbols from the first part of theinterleaver memory on to the sub-carrier signals of the even OFDMsymbols in accordance with an order defined by the set of addressesgenerated with one of the permutation codes of the sequence, writing thesecond set of input data symbols into a second part of the interleavermemory in accordance with a sequential order of the second sets of inputdata symbols, and reading out the second sets of input data symbols fromthe second part of the interleaver memory on to the sub-carrier signalsof the odd OFDM symbols in accordance with an order defined by the setof addresses generated with another of the permutation codes of thesequence.
 9. A transmitter for transmitting data using OrthogonalFrequency Division Multiplexing (OFDM), the transmitter including a dataprocessing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbol, the dataprocessing apparatus comprising an interleaver operable to read-into aninterleaver memory the predetermined number of data symbols for mappingonto the OFDM sub-carrier signals, and to read-out of the interleavermemory the data symbols for the OFDM sub-carriers to effect the mapping,the read-out being in a different order than the read-in, the orderbeing determined from a set of addresses, with the effect that the inputdata symbols are interleaved on the sub-carrier signals, an addressgenerator operable to generate the set of addresses, an address beinggenerated for each of the input data symbols to indicate one of thesub-carrier signals onto which the data symbol is to be mapped, theaddress generator comprising a linear feedback shift register includinga predetermined number of register stages and being operable to generatea pseudo-random bit sequence in accordance with a generator polynomial,a permutation circuit operable to receive the content of the shiftregister stages and to permute the bits present in the register stagesin accordance with a permutation code to form an address of one of theOFDM sub-carriers, and a control unit operable in combination with anaddress check circuit to re-generate an address when a generated addressexceeds a predetermined maximum valid address, characterised in that thepredetermined maximum valid address is less than one thousand and twentyfour, the linear feedback shift register has nine register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[8]=R′_(i-1)[0]⊕R′_(i-1)[4], and the permutation order forms, withan additional bit, a ten bit address R_(i)[n] for the i-th data symbolfrom the bit present in the n-th register stage R′_(i)[n] in accordancewith a code defined by the table: R′_(i) bit positions 8 7 6 5 4 3 2 1 0R_(i) bit positions 4 3 2 1 0 5 6 7 8


10. A transmitter as claimed in claim 9, wherein the transmitter isoperable to transmit data in accordance with a Digital VideoBroadcasting standard such as the Digital VideoBroadcasting-Terrestrial, Digital Video Broadcasting-Handheld standard,the Digital Video Broadcasting-Terrestrial2 standard or the DigitalVideo Broadcasting Cable2 standard.
 11. A method of mapping input datasymbols to be communicated onto a predetermined number of sub-carriersignals of an Orthogonal Frequency Division Multiplexed (OFDM) symbol,the method comprising reading-into an interleaver memory thepredetermined number of input data symbols for mapping onto the OFDMsub-carrier signals, reading-out of the memory the input data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the input data symbols areinterleaved on the sub-carrier signals, generating the set of addresses,an address being generated for each of the input data symbols toindicate one of the sub-carrier signals onto which the data symbol is tobe mapped, the generating the set of addresses comprising using a linearfeedback shift register including a predetermined number of registerstages to generate a pseudo-random bit sequence in accordance with agenerator polynomial, using a permutation circuit operable to receivethe content of the shift register stages to permute the bits present inthe register stages in accordance with a permutation code to form anaddress, and re-generating an address when a generated address exceeds apredetermined maximum valid address, characterised in that thepredetermined maximum valid address is less than one thousand and twentyfour, the linear feedback shift register has nine register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[8]=R′_(i-1)[0]⊕R′_(i-1)[4], and the permutation code forms, withan additional bit, a ten bit address R_(i)[n] for the i-th data symbolfrom the bit present in the n-th register stage R′_(i)[n] in accordancewith a code defined by the table: R′_(i) bit positions 8 7 6 5 4 3 2 1 0R_(i) bit positions 4 3 2 1 0 5 6 7 8


12. A method as claimed in claim 11, wherein the predetermined maximumvalid address is a value substantially between seven hundred and onethousand and twenty four.
 13. A method as claimed in claim 11, whereinthe OFDM symbol includes pilot sub-carriers, which are arranged to carryknown symbols, and the predetermined maximum valid address depends on anumber of the pilot sub-carrier symbols present in the OFDM symbol. 14.A method as claimed in claim 11, wherein the reading-into theinterleaver memory the input data symbols, and the reading-out of theinterleaver memory the input data symbols for mapping onto the OFDMsub-carrier signals to effect the mapping, includes for even OFDMsymbols, reading in the data symbols according to the set of addressesgenerated by the address generator and reading out in a sequentialorder, and for odd OFDM symbols, reading in the symbols into theinterleaver memory in a sequential order and reading out the datasymbols from the interleaver memory in accordance with the set ofaddresses generated by the address generator.
 15. A method as claimed inclaim 11, wherein the using a permutation circuit to receive the contentof the shift register stages and permuting the bits present in theregister stages in accordance with a permutation code to form anaddress, includes changing the permutation code, which permutes theorder of the bits of the register stages to form the addresses, from oneOFDM symbol to another.
 16. A method as claimed in claim 15, wherein thechanging the permutation code, which permutes the order of the bits ofthe register stages to form the addresses, from one OFDM symbol toanother includes cycling through a sequence of different permutationcodes for successive OFDM symbols.
 17. A method as claimed in claim 16,wherein the sequence of permutation codes comprises two permutationcodes, which are: R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bitpositions 4 3 2 1 0 5 6 7 8

and R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 3 2 5 0 14 7 8 6


18. A method as claimed in claim 16, wherein the sub-carriers of theOFDM symbols are half or less than half a maximum number of sub-carriersin the OFDM symbols of any of a plurality of operating modes, the methodcomprising dividing the input data symbols into first sets of input datasymbols for mapping onto even OFDM symbols and second sets of input datasymbols for mapping onto odd OFDM symbols, and interleaving the inputdata symbols from both first and second sets in accordance with an oddinterleaving process comprising writing the first sets of input datasymbols into a first part of the interleaver memory in accordance with asequential order of the first sets of input data symbols, reading outthe first sets of input data symbols from the first part of theinterleaver memory on to the sub-carrier signals of the even OFDMsymbols in accordance with an order defined by the set of addressesgenerated with one of the permutation codes of the sequence, writing thesecond set of input data symbols into a second part of the interleavermemory in accordance with a sequential order of the second sets of inputdata symbols, and reading out the second sets of input data symbols fromthe second part of the interleaver memory on to the sub-carrier signalsof the odd OFDM symbols in accordance with an order defined by the setof addresses generated with another of the permutation codes of thesequence.
 19. A method of transmitting input data symbols via apredetermined number of sub-carrier signals of an Orthogonal FrequencyDivision Multiplexed (OFDM) symbol, the method comprising receiving apredetermined number of the input data symbols for mapping onto thepredetermined number of sub-carrier signals, reading-into an interleavermemory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, reading-out of the interleaver memory the datasymbols for the OFDM sub-carriers to effect the mapping, the read-outbeing in a different order than the read-in, the order being determinedfrom a set of addresses, with the effect that the input data symbols areinterleaved on the sub-carrier signals, generating the set of addresses,an address being generated for each of the input data symbols toindicate one of the sub-carrier signals onto which the input data symbolis to be mapped, the generating the set of addresses comprising using alinear feedback shift register including a predetermined number ofregister stages to generate a pseudo-random bit sequence in accordancewith a generator polynomial, using a permutation circuit operable toreceive the content of the shift register stages to permute the bitspresent in the register stages in accordance with a permutation order toform an address, and re-generating an address when a generated addressexceeds a predetermined maximum valid address, characterised in that thepredetermined maximum valid address is less than one thousand and twentyfour, the linear feedback shift register has nine register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[8]=R′_(i-1)[0]⊕R′_(i-1)[4], and the permutation order forms, withan additional bit, a ten bit address R_(i)[n] for the i-th data symbolfrom the bit present in the n-th register stage R′_(i)[n] in accordancewith a code defined by the table: R′_(i) bit positions 8 7 6 5 4 3 2 1 0R_(i) bit positions 4 3 2 1 0 5 6 7 8


20. A method of transmitting as claimed in claim 19, wherein thetransmitting is in accordance with a Digital Video Broadcasting standardsuch as the Digital Video Broadcasting-Terrestrial, Digital VideoBroadcasting-Handheld standard, the Digital VideoBroadcasting-Terrestrial2 standard or the Digital Video BroadcastingCable2 standard.
 21. An address generator for use with transmission ofdata symbols interleaved onto sub-carriers of an Orthogonal FrequencyDivision Multiplexed symbol, the address generator being operable togenerate a set of addresses, each address being generated for each ofthe data symbols to indicate one of the sub-carrier signals onto whichthe data symbol is to be mapped, the address generator comprising alinear feedback shift register including a predetermined number ofregister stages and being operable to generate a pseudo-random bitsequence in accordance with a generator polynomial, a permutationcircuit operable to receive the content of the shift register stages andto permute the bits present in the register stages in accordance with apermutation order to form an address, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address,characterised in that the predetermined maximum valid address is lessthan one thousand and twenty four, the linear feedback shift registerhas nine register stages with a generator polynomial for the linearfeedback shift register of R′_(i)[8]=R′_(i-1)[0]⊕R′_(i-1)[4], and thepermutation order forms, with an additional bit, a ten bit addressR_(i)[n] for the i-th data symbol from the bit present in the n-thregister stage R′_(i)[n] in accordance with a code defined by the table:R′_(i) bit positions 8 7 6 5 4 3 2 1 0 R_(i) bit positions 4 3 2 1 0 5 67 8